Xtestpicture creates a full-screen picture that is intended to help
you adjusting your monitor, resembling the usual TV test pictures.

The picture consists of some geometrical elements, like two crossing
diagonals, a circle, a couple of rectangles.  In the center, there's
a rectangle filled with vertical lines, to prove sharpness and video
bandwidth, and a color area to adjust the color saturation and contrast.

That's about all there is to be, doesn't sound very impressive, right?
Simply click a mouse button to get rid of it again.
